Have your fish store grind the fillets or grind them yourself in a food processor, using on-and-off pulses, or in a meat grinder. Do not puree.
Cook onion in oil in a large skillet over medium-low heat until soft, 6 to 10 minutes.
Combine fish, onion, water, eggs, matzoh meal, sugar, parsley, salt and pepper in large bowl of electric mixer. Blend together on low speed, then beat at medium speed for 15 minutes, scraping down sides of bowl frequently.
Preheat oven to 325°F. Oil an 8-inch (6-cup) Bundt pan.
Spoon fish mixture into prepared pan, smoothing top. Cover tightly with aluminum foil. Set pan in larger pan. Place on oven shelf; add boiling water to larger pan to 1-inch depth.
Bake in preheated 325°F oven for 1 hour or until set and knife inserted in center comes out clean. Remove pan to wire rack to cool for 15 minutes. Gently slide a small knife around edge to loosen; invert on serving platter. Cover and refrigerate until ready to serve.
Cut into slices and serve with horseradish.